Stator for electric motor

ABSTRACT

The invention relates to a stator with a plurality of field coils, wherein the yoke teeth, which bear the field coils, of the poles are connected with their free ends to a stator ring, and wherein adjacent pole shoes or poles are connected to one another.

The present invention [relates to] an embodiment of a stator for apolyphase machine according to the preamble of claim 1.

PRIOR ART

External rotor motors according to the prior art have a stator which iswound with coils, and also a rotor with embedded or adhesively bondedmagnets. Pole shoes are generally provided in order, on the one hand, toreduce the detent torque of the motor and, on the other hand, to ensurethat the coils are fixed in the stator. Pole shoes also have anadvantageous effect on magnetic properties, as the flux of the permanentmagnets can be conducted more effectively into the stator.

The advantages of the pole shoes lead to drawbacks in the winding of thestator. The needle or flyer winding is most widespread. Needle windingrequires a needle which is guided in the groove in the yoke tooth andgenerally has a width of 2.5-3 times that of the wound wire. Motors forthe low-voltage range, in particular, require thick wires in order toachieve a low number of turns and inductance. On account of the requiredwidth of the needle of the needle winder, a winding space correspondingto the diameter of the needle is lost during winding of the stator.Needle winding technology therefore generally allows only low copperfill factors of approx. 30-35% to be achieved. In order to achievebetter fill factors, use is made of individual teeth which havedovetails and are connected in a form-fitting manner to a stator ring.The individual teeth are then wound individually and inserted into thestator. The drawback of the individual teeth is the high handlingcomplexity. In addition, individual-tooth windings require a punchedgrid to which the coils are interconnected. The punched grid is acost-pusher and should therefore be simplified.

In order to increase the copper fill factor, EP 0871282 A1 discloses aone-part stator having bending zones in the region of the return. Thisallows the stator to be wound in the straight state. After the winding,the stator is bent into a shape. This technique has the advantage of lowmaterial wastage and allows a high copper fill factor in internal rotormotors. However, a high fill factor cannot be achieved in external rotormotors (FIG. 49, EP 0871282 A1). In addition, the pole shoes areconfigured in a very complex manner in order to allow the deactivatingof individual coils with an insulating body, as illustrated in FIGS. 13and 14 of EP 0871282 A1.

OBJECT AND CARRYING-OUT OF THE INVENTION

The aim of the invention is to provide a stator for an electric motorwith which, on the one hand, a high fill factor can be achieved and, onthe other hand, the handling complexity and the costs can be reduced.Furthermore, the stator construction should have an advantageous effecton the detent torque of the motor and thus reduce the formation ofnoise.

This object is inventively achieved by a two-part stator constructionhaving the features of claim 1 and has the following advantages:

-   -   optimum configuration of the pole shoes with regard to        minimising the detent torque;    -   high flexibility as a result of the use of individual coils or        alternatively the entire stator winding; achieving a high fill        factor for external and internal rotor motors;    -   low material wastage and handling complexity;    -   low tolerance sensitivity of the stator parts;    -   reduction of shunts by way of cut-outs;    -   low manufacturing costs.

Further advantageous configurations of this stator and of acorresponding electric motor emerge from the features of the sub-claims.

Various configurations of the drive according to the invention will bedescribed hereinafter with reference to the drawings, in which:

FIG. 1 shows stator construction of a polyphase machine according to theprior art;

FIG. 2 shows stator construction according to the invention for externalrotor motor in the assembled state;

FIG. 3 a shows stator construction according to the invention forexternal rotor motor as an individual part for winding coils;

FIG. 3 b is a plan view onto a stator core as an individual part;

FIGS. 3 c-3 f show various embodiments of the connecting webs of theyoke teeth;

FIG. 4 shows stator construction according to the invention for internalrotor motor;

FIG. 5 shows alternative toothed connection with plastics materialencapsulation;

FIG. 5 b is a 3D sectional illustration of alternative toothedconnection with plastics material encapsulation;

FIG. 6 shows linear stator made up of a plastics material ladder withinserted individual teeth;

FIG. 7 shows winding tool receptacle for stator of an external rotormotor;

FIG. 7 a shows winding tool receptacle for stator of an internal rotormotor;

FIG. 8 shows assembled stator of an external rotor motor with plasticsmaterial encapsulation; and

FIG. 9 shows assembled stator of an external rotor motor with pliableconnecting zones made of metal with form-fitting connection to statorring.

FIG. 1 shows a cross section through a stator construction according tothe prior art. The lower part of the picture shows a one-part statorhaving poles 1, a pole shoe formation 1 b and a transverse flux region 1a. The excitation coils 2 are slid onto the stator. The dead space 2 a,which cannot be wound, is positioned between the poles 1 and is obtainedfrom the guidance of the needle and the width of the needle of theneedle winder. The flux generated by the excitation coils 2 is closedvia the permanent magnets 3 and the rotor 6. The upper part of thepicture shows an alternative to the lower stator configuration. There,the stator is made up of a plurality of parts and consists of a statorring 7 and a plurality of individual poles 4 which are individuallywound with coils 2. The individual poles 4 are fixed in the stator ring7 via a dovetail 4 b. Generally, the stator cores are held together viaa press fit and caulking. This embodiment of the stator requiresstringent demands to be placed on the dimensional stability of thelaminated cores for the individual poles 4 and the ring 7, as a resultof which the production costs are high. The stator is connected to thecarrier 5, which is a part of the motor housing, via grooves or via apress fit.

FIG. 2 shows the stator construction according to the invention for anexternal rotor motor. The stator consists of 2 parts: a stator ring 7and a stator 8 bent into a round shape. The stator 8 has dovetails 13 aand a connecting web 8 b connecting the individual yoke teeth 8 c.

The stator is wound with excitation coils 2 in unrolled form. The statoras a whole is thus insulated and wound before it is bent and attached tothe stator ring 8. It is particularly advantageous in this regard thatthe stator can be wound as a whole, i.e. a plurality of coils of a phasecircuit is wound onto the stator in one step. This eliminates the needfor individual coil winding. This greatly simplifies the punched gridconnecting the individual coils, as fewer coil wires are to be contactedin a series winding of the stator. In addition, the demands placed onthe precision of the dimensional stability of the dovetails are lessstringent compared to the prior art (FIG. 1), as the stator core is acoherent whole and it is not necessary for each tooth to achieve anoptimum press connection between the tooth and stator ring. Shoulders 13b, 13 c and 13 d and 13 e, which merely ensure positioning of the statorin the ring, are therefore conceivable alternatives to the dovetail. Thestator can then be connected in a form-fitting manner to the stator ring7 by end-side welding 16 in the region of the connection between thestator 8 and stator ring 7. In order to save material costs, aconfiguration of the stator, inner ring 7 as a flexible part ispossible. In order to ensure this, appropriate bending zones 7 a areprovided in the stator inner ring. The stator inner ring can then be cutout, using little material, as a straight part from a lamination stripand be connected to the stator carrier 5 via appropriate form-fittingconnections (not shown).

The stator form is configured in such a way that an overlap region 11 isobtained in the region of the centre of the yoke tooth 8 c or pole shoe8 a after the bending and fastening to the stator ring 7. An appropriatespacing 12 is provided so that the tolerance sensitivity of the statorcan be reduced. There, the stator is for example closed by a weld seam10.

A form-fitting connection (not shown) of the bending stator, by means ofwhich the bending stator is closed and held together, is also possibleas an alternative to the weld seam. In order to reduce eddy currentlosses, the free end 10 can also be connected to the adjoining tooth byadhesive bonding or caulking. The overlap region is advantageous at thecentre of the yoke tooth 8 c, as sufficient material is present for theweld seam and the magnetic properties are not markedly impaired.

The connecting web 8 b between the poles 8 has an adverse effect on themagnetic properties, as a magnetic short circuit occurs. The walls ofthe connecting zone should therefore be made as thin as possible so thatthe connecting web soon reaches saturation on excitation and themagnetic resistance is thus increased. However, at a different point,the closed stator has a positive effect on the motor properties, as thedetent torque generated by the permanent magnets 3 is greatly reduced,as the magnets face a homogeneous, continuous stator contour. As aresult, the tightening effect acting on the poles 8 is greatly weakenedby the magnetisation of the magnets 3. This has an advantageous effecton the noise and also the smooth-running rotatability of the rotor inthe currentless state. The latter property is particularly important inuse in electric power steering, as a very low detent torque is required.

FIG. 3 a shows the stator 8 in unrolled form. The connecting zone 8 bbetween the webs must be designed in such a way that the geometry of thematerial is kept appropriately stretched as a result of the bending.Thus, it may be advantageous for the geometry of the connecting web tohave a defined bending/buckling zone or for the connecting region 8 b tohave a uniform thickness and for a secant thus to be formed as a resultof bending. Furthermore, FIG. 3 a shows an advantageous winding of thestator with excitation coils. The coils 2 are wound onto a first toothvia the phase access 2 b; the wire is then extended further to the nexttooth via the connection 2 c until the complete phase is wound and endsat the output 2 d. The output 2 d is the outlet to the star connection.This winding technology greatly reduces the number of contacts, as aplurality of coils is wound in series. This allows the punched grid tobe greatly simplified. In addition, fewer contacts lead to less wastagein production. It is also conceivable for the coils to be wound inseries in a device and then to be attached to the stator. This ispossible as the poles 8 are designed to be straight and the [ . . . ] atthe free ends 8 f of the yoke teeth is freely accessible. An externalwinding is advantageous, as the coils can be wound on anindividual-tooth winder machine which is more economical than a needlewinding machine. In addition, the wire can be wound in a moredimensionally accurate manner via a specific device. This allows, inturn, a higher copper fill factor.

FIG. 3 b is a plan view onto the stator of a variant of the laminatecores from FIG. 3 a. Thus, it is possible to assemble the stator fromthe same laminate cores and to achieve a continuous connecting web 8 bbetween the poles 8. This is particularly advantageous for reducing thedetent torque, but leads to a magnetic short circuit and thus to lossesin the development of forces.

In so far as the detent torque is therefore not particularly important,it is advantageous for the stator core to be composed of a composite ofindividual-tooth laminations 9 b and continuous laminations 9 a. It isthus possible to reduce the proportion of the height of the stator corethat is made up by the connecting webs; this reduces the magnetic shortcircuit between the poles while at the same time allowing a one-partstator. The stator cores can be connected to one another by welding 14,by adhesive bonding or punch coring. Alternatively, it is alsoconceivable for the stator to be punched out in the region 15, so thatthe magnetic short circuit is minimised between the stator teeth.

FIG. 3 c to FIG. 3 f show various variants of the connection between thestator and stator ring and various zones of transition between theindividual yoke teeth 8 c. In FIG. 3 c, the transition region 8 c isdesigned so as to be continuous and have a uniform thickness. In thiscase, the connecting region 8 d is not designed for minimum thickness.This reduces the demands placed on the punching tool. However, themagnetic shunt can be effectively minimised by appropriate cut-outsbetween the yoke teeth. The bending is carried out on the zones 17; asecant is formed in the bent state. In FIG. 3 d, the connecting regionis designed in a very thin-walled manner. This is expedient if cut-outsare to be dispensed with. In this case, the minimum thickness is thatwhich can be economically achieved using a punching tool; generally, thethickness of the connecting region should be between 0.5 and 1 mm. InFIG. 3 e, the pole shoe tapers toward a defined bending zone 8 f at 2angles. This allows a defined bending zone. FIG. 3 f is designed in asimilar manner, with the difference that the connecting region betweenthe poles has a uniform wall in a region 8 h and is then tapered via adefined bending zone 8 g. Which embodiment is most suitable in theindividual case depends on the specific design of the motor. Theimportant thing is that the connecting region between the poles is verythin-walled, or the magnetic short circuit is reduced by cut-outsbetween the poles.

FIG. 4 shows a similar stator for an internal rotor motor. The statorconsists, again, of 2 parts. The first part of the stator core 18 withyoke teeth 18 c, poles 18 a, a dovetail 18 s and a thin-walledconnecting zone 18 b carries the excitation coils 2. This part isconnected to the stator ring 9 via dovetails or shoulders 18 s which arecomparable to the connections shown in FIG. 2. The permanent magnets 3,which are adhesively bonded or embedded on the rotor 6, face the poles18 a. In this case, the stator can be brought from a straight unwoundform (cf. FIG. 3 a) into the corresponding round form by bending roundand connecting the stator ends (2 s) or, alternatively, be manufacturedas a punched core in round form without bending. In the latter variant,it is advantageous for the excitation coils 2, as illustrated in FIG. 3a, to be wound beforehand and attached to the stator. Direct winding ofthe stator is also possible. A higher fill factor is possible in thiscase too, as better filling of the needle of the needle winder ispossible. It is also advantageous for the stator to have, in the regionof the connecting webs 18 b, an appropriate bending zone 18 d (cf. withFIGS. 3 c-3 d). This allows the stator to be manufactured in straightform with minimal material wastage and to be shaped by bending back. Thefree ends 11 are overlapped with one another and welded to one anotherby means of a weld seam 10. In the bent-back state, the stator can thenbe optimally wound on an appropriate device with a needle winder. Inorder to save material costs, a configuration of the stator outer ring 9as a flexible part is possible. In order to ensure this, appropriatebending zones 9 a are provided in the stator outer ring 9. The statorouter ring can then be cut out, using little material, as a straightpart from a lamination strip and be connected to the bending stator 18via form-fitting connections 18 a.

FIG. 5 a shows an alternative embodiment in which the poles areconnected to one another using plastics material. In this case, in oneembodiment, the individual teeth 19 are encapsulated in a suitabledevice in a form such that all the teeth of the stator are connected toone another. The encapsulation 20, on the one hand, insulates the toothfor the winding 20 a and has a pliable zone 20 b in the connections ofthe individual teeth. In order to ensure greater flexibility, thepliable zone 20 b can be embodied in an undulatory manner, in particularwith a loop 20 c, thus improving the elasticity of the bending. The base19 a of the tooth has an appropriate polygonal geometry, in particularwith projections 19 b and/or undercuts 19 c which extend in the axialdirection and allow a form-fitting connection to the stator inner ring7. An axially extending shoulder or web 20 d, which ensures that thecoils (20 a) are fixed in the appropriate form during winding and do notslip off from the carrier, is provided for the purposes of winding. Thistype of connection can be provided both for a stator of an internalrotor motor (FIG. 4) and for a stator of an external rotor motor (FIG.2).

FIG. 5 b shows a 3D section through the encapsulation. In theencapsulation, it is advantageous for an encapsulation 20 e to beprovided at the end side. It is beneficial to design the end-sideencapsulation in such a way that the excitation coils are guided andfixed during winding in plastics material guides which are provided.

FIG. 6 shows an alternative to plastics material encapsulation. In thisembodiment, the individual teeth 19 are inserted into a prefabricatedplastics material part in the form of a ladder 21. The plastics materialladder 21 serves, on the one hand, as a stator insulator; on the otherhand, it, like the plastics material encapsulation illustrated in FIG. 5a, has a pliable zone 21 a. A configuration of the plastics materialladder 21 that allows the teeth 19 to be clipped into pockets 21 b isadvantageous. The clipping-in can be carried out by means of locks (notshown) which can be formed by appropriate undercuts, projections and/ortongues on the parts 19 and 21. The ladder solution can be used for thestator design of both an internal and an external rotor motor.

The plastics material connection variants illustrated in FIGS. 5-6 havethe advantage that it is necessary to produce only stator teeth 19 andnot lamination strips for the connecting webs. The manufacture of theteeth and also the stator coring process are thus much more economical.Drawbacks include the increased demands placed on the form-fittingconnection of the bending stator to the stator ring 7, 9, as the bendingstator cannot be efficiently held together by the plastics materialconnections.

The stator can then be wound by a linear winding machine or be bent backand wound onto an appropriate Stator winding tool. The bending zone 20b, 20 c, 21 a must therefore be designed in such a way as to be able toensure a first bend onto the carrier of the winding machine and also asecond band in the opposite direction for insertion into the statorcarrier ring. The stator fastened to the winding tool 22 is shown inFIG. 7. The teeth of the bent-back stator are tightened using magneticforce, for example using permanent magnets 23, for the purposes ofwinding and if appropriate fixed in the winding tool 22 via anadditional clamping device (not shown).

FIG. 7 a shows an appropriate winding tool for the stator of an internalrotor motor. In order to achieve a very good fill factor, it isbeneficial for the stator 18 to be spread via a winding mandrel 25. Theplastics material connecting webs, which are advantageously designed viaan extensible loop 20 c, are then extended, thereby creating theappropriate clearance for the needle of the needle winder. This allowsthe fill factor to be advantageously increased during automated winding.An elliptical configuration of the winding mandrel, which is rotatedduring winding, is also possible. As a result, the connecting zonebetween the teeth is also stretched and an appropriate clearance isproduced for the guidance of the needle.

Once the encapsulated bending stator 20 or the individual teeth 19embedded into a plastics material ladder 21 are wound, the bendingstator is connected in a form-fitting manner to the stator ring 7 via asuitable device. For this purpose, the teeth 19 are inserted in theaxial direction with their ends 19 a into the appropriate recesses inthe stator ring 7. The assembled stator is shown in FIG. 8.

FIG. 9 shows, the form-fitting connection variant of the bending statorwith apertures without plastics material. The form-fitting connection 7v, 19 a of the teeth 19 to the inner ring 7 imparts sufficient stabilityto the individual teeth 19. A connection, in particular a welding or aform-fitting connection, of the free ends 8′, 8″ of the bending stator 8in the circumferential direction at O, as in the embodiment illustratedin FIG. 2, may therefore be dispensed with.

1. Stator with a plurality of excitation coils (2), wherein the yoketeeth (8 c; 18 c; 19 z), which carry the excitation coils (2), of thepoles (8; 18 a; 19) are connected with their free ends (8 f; 18 s; 19 a)to a stator ring (7; 17), and wherein adjacent pole shoes (8 a, 19 s) orpoles (18 a) are connected (8 b, 8 d, 8 e, 8 f, 9 a; 18 d; 20 b; 24) toone another.
 2. Stator according to claim 1, characterised in that theradial material thickness of the pole shoes (8 a, 19 s) decreases in thecircumferential direction, starting from the pole teeth (8 c, 19 z) upto the connecting regions (ft, 8 d, 8 e, 8 f, 9, 20 b, 21 a, 24). 3.Stator according to claim 1 or 2, characterised in that the connectingregions (ft, 8 d, 8 e, 8 f, 24) have material cut-outs, in particularwindow-like cut-outs (15, 25), or window-like cut-outs are arrangedbetween the connecting regions (9 a).
 4. Stator according to one ofclaims 1 to 3, characterised in that the yoke teeth (8 c, 19 z) and poleshoes (8 a, 19 s) are formed by stacked laminations.
 5. Stator accordingto claim 4, characterised in that the connecting regions (8 b) areformed by the laminations forming the yoke teeth (8 c) and pole shoes (8a).
 6. Stator according to claim 4 or 5, characterised in that at leastone lamination (9 b) does not have any connecting regions (8 b, 8 d, 8e, 8 f) or the laminated core has laminations having merely two yoketeeth (8 c), pole shoes (8 a) and a connecting region (8 b) connectingthe pole shoes (8 a) to one another.
 7. Stator according to claim 6,characterised in that the laminations are embodied and arranged relativeto one another in such a way that they connect in an alternating ordertwo adjacent pole shoes to one another by connecting regions and formwindow-like openings in the axial direction therebetween.
 8. Statoraccording to one of claims 2 to 7, characterised in that the connectionsbetween the pole shoes (8 a) are formed by individual laminations (9 a)with connecting regions (8 b, 8 d, 8 e, 8 f, 8 h) between whichlaminations (9 b) without connecting regions, which form the window-likeopenings (15), are arranged axially.
 9. Stator according to one ofclaims 1 to 4, characterised in that the poles (19) and/or pole shoes(19 z) are connected to one another by means of connecting regions (20b, 21 a, 24) formed from plastics material.
 10. Stator according toclaim 9, characterised in that the poles (19) are sheathed, inparticular encapsulated, at least partly or wholly with plasticsmaterial (20, 21) and the connecting regions (20 b, 21 a, 24) are formedby the plastics material (20, 21).
 11. Stator according to claim 9 or10, characterised in that the poles (19) lie in a plastics material part(21), in particular lie in pockets (21 b).
 12. Stator according to claim11, characterised in that the poles (19) are fastened in the pockets bymeans of locks.
 13. Stator according to one of claims 9 to 12,characterised in that the plastics material sheathing (20, 21) hasprojections for fixing the stator winding (20 a), which projections areformed in particular by axial webs or projections (20 d).
 14. Statoraccording to one of claims 9 to 13, characterised in that the radial endfaces (19 st) of the pole shoes (19 s) are not covered with plasticsmaterial (20).
 15. Stator according to one of the preceding claims,characterised in that the connecting regions have buckling or bendingregions (8 d, 8 e, 8 f, 8 g, 8 h, 20 c) or are embodied as buckling orbending regions.
 16. Stator according to one of the preceding claims,characterised in that the laminations forming the yoke teeth (8 c), poleshoes (8 a) and connecting regions (8 b) are circularly bentlaminations, the end sides (2 s) of which adjoin one another or overlap.17. Stator according to claim 15, characterised in that the end sides (2s) are connected to one another, in particular welded, adhesively bondedor caulked to one another.
 18. Stator according to claim 15 or 16,characterised in that the overlap (11) of the end sides (2 s) isarranged in the region of a yoke tooth (8 c) or pole shoe (8 a). 19.Stator according to one of the preceding claims, characterised in thatthe yoke teeth (8 c, 19 z) of the poles (8, 19) lie with their free ends(13 a, 13 b, 13 c, 19 a) in grooves in the stator ring (7).
 20. Statoraccording to one of claims 1 to 18, characterised in that the statorring (7) has radial projections (13 e) which engage with correspondingcut-outs, in particular grooves in the free ends (8 f) of the yoke teeth(8 c).
 21. Stator according to one of the preceding claims,characterised in that the connecting region (8 b, 20 b, 21 a) betweenthe pole shoes (8 a, 19 s) is embodied in a thin-walled manner. 22.Stator according to one of the preceding claims, characterised in thatthe laminated core is connected, in particular welded or caulked, to thestator ring at the end side.
 23. Stator according to one of thepreceding claims, characterised in that the connecting regions (8 b, 8d, 8 e, 8 f, 9, 20 b, 20 c, 21 a, 24) are embodied in a flexible and/orspringy manner.
 24. Stator according to one of the preceding claims,characterised in that the stator ring (7, 9) has bending zones orresilient regions (7 a, 9 a).
 25. Method for manufacturing a statoraccording to one of the preceding claims, characterised in that thelaminated core of the stator is wound in unrolled form and afterwardsthe laminated core is bent in a cylindrical manner, after which the freeend sides of the laminations are then connected to one another and thewound laminated core is slid onto the stator ring (7).
 26. Method formanufacturing a stator according to one of the preceding claims,characterised in that, prior to the winding-on or sliding-on of theexcitation windings (2), window-like openings (15) are made, inparticular punched or cut out, in the connecting regions (8 b) of thelaminated core.
 27. Method according to claim 25 or 26, characterised inthat the free end sides (2 s) are made to overlap (11) and welded,adhesively bonded or caulked to one another.
 28. Method formanufacturing a stator according to one of claims 1 to 24, characterisedin that the poles (19) are composed of laminations and positioned in aninjection mould and encapsulated with plastics material and connected toone another, after which the stator winding is then attached to the poleteeth (19 z) and in a subsequent working step the poles are slid onto astator ring (7).
 29. Method for manufacturing a stator according to oneof claims 1 to 24, characterised in that the poles (19) are composed oflaminations and inserted and locked in pockets of a plastics materialpart (21), after which the stator winding is then attached to the poleteeth (19 z) and in a subsequent working step the poles are slid onto astator ring (7).
 30. Method for manufacturing a stator according toclaim 28 or 29, characterised in that, prior to the winding-on of thestator winding (20 a), the stator is bent in such a way as to produce arelatively large free space between the yoke teeth (8 c; 18 c; 19 z).31. Method for manufacturing a stator according to claim 30,characterised in that the bent stator is mounted onto a winding mandrel,in particular is held on the winding mandrel (22) by means of magneticforces (23).
 32. Method for manufacturing a stator according to claim 30or 31, characterised in that the winding mandrel (22) radially spreadsthe stator during the winding process, the connecting regions (20 b, 20c, 21 a) being embodied in an extensible manner so that the poles arenot deformed during spreading.
 33. Polyphase machine with a statoraccording to one of the preceding claims.
 34. Polyphase machineaccording to claim 33, characterised in that the polyphase machine has arotor equipped with permanent magnets.
 35. Polyphase machine accordingto claim 33 or 34, characterised in that the polyphase machine isembodied as an internal or external rotor, the stator ring (7) beingsurrounded by the laminated stator core in the external rotor and thelaminated stator core being surrounded by the stator ring (9) in theinternal rotor.